This invention relates to an optical deflector used for a laser beam printer.
In general, an optical deflector is provided with a motor to rotate a polygonal mirror for deflecting a laser beam emitted by a semiconductor laser diode. In such a motor, friction between a motor shaft and bearings needs be minimized for high speed rotation. To meet this requirement, the motor shaft is radially supported by journal bearings of a dynamic pressure type, and is floated or suspended to be axially supported by an attractive magnetic force produced between permanent magnets fixed individually to an end of the motor shaft and a motor housing. Such an arrangement is disclosed in U.S. Pat. No. 4,443,043 to Yamaguchi.
In the motor mentioned above, air is introduced into a gap of 3 .mu.m to 6 .mu.m between the bearing members of dynamic journal bearings of herringbone type through herringbone grooves. As a result, a bearing pressure within the gap is increased at the center portion of the bearings, so that the motor shaft is subjected to a radial force. The motor shaft is suspended in a position in which the thrust load and the attractive magnetic force produced between the permanent magnets are balanced. This rotary mechanism with axial bearings and thrust bearings permits the motor to rotate at a high speed, i.e., in the range of 4,000 rpm to 15,000 rpm.
For producing the attractive magnetic force strong enough to balance with the thrust load, however, the number of magnets must be increased and the gap between the magnets must be reduced. Reduction of the gap is restricted by the limit to which the magnets can be machined. Also, the unevenness of the radial attractive magnetic force causes the rotation of the motor shaft to become irregular. As a result, the scanning position of a laser scanned by the polygonal mirror varies.